The present invention relates to an arrangement for protecting (shielding) one or more electromagnetic sensors against undesirable electromagnetic radiation.
In many connections there is a desire to protect (shield) one or more electromagnetic sensors, for example antennas, from undesirable electromagnetic radiation and mechanical influences. Very often it is desirable to protect the sensor's sensitive electronics from interfering and/or destructive signals and to influence/reduce/alter the radar signature of the sensor. An object's radar signature is the object's three-dimensional electromagnetic radiation diagram which is obtained from it being irradiated with electromagnetic signals of differing frequencies and from the object's self-produced electromagnetic radiation. An object's radar signature can also be seen as a diagram of the object's equivalent reflection surface in three dimensions, which of course only gives a picture of the radar signature for incident radiation.
One way of achieving a protector/shield which fulfils the aforementioned desire is by using a frequency selective surface/radome (FSS--Frequency Selective Surface, RADOME --RAdar DOME).
In the construction of frequency-selective surfaces, periodic patterns consisting of aperture elements can be used, for example slits which, simply expressed, are holes of some form in an electrically-conductive plane, or periodic patterns can be used which consist of electrically-conductive elements, for example dipoles or printed conductive elements, which can be described as electrically-conductive islands in some form on an insulated plane. These two types of periodic pattern give rise to surfaces with different frequency behaviour.
If aperture elements are used, the surface transmits at the aperture elements' resonant frequency f.sub.0. This means that the surface will have a transmission window (compare with pass-band filters) where the maximum transmission through the surface is obtained at a frequency which is determined by the elements' resonant frequency with a wavelength .lambda. (element length.about..lambda./2{.lambda.=electrical wavelength}). If on the other hand, conductive elements are used, the surface will reflect at the elements' resonant frequency. That means that the surface will have a reflection window (compare with suppression/stop-band filters) where the maximum reflection is obtained at a frequency which is determined by the elements' resonant frequency (element length.about..lambda./2).
The natural choice when constructing a frequency-selective surface/radome is a periodic pattern with aperture elements. Such a surface has a transmission window, i.e. it is transparent for a chosen frequency range. By combining two or more such layers with a space between the layers, the characteristics of the radome can be additionally improved/altered, i.e. full transmission at a desired transmission frequency can be obtained, as well as steeper flanks in the window.
This type of construction has several disadvantages however, partly due to the fact that a radome with such a surface is electromagnetically open (has a pass-band) at multiples of the resonant frequency (nf.sub.0, n=1, 2, . . . ), but also due to the fact that the periodic pattern gives rise to grating lobes for frequencies from about 1.5f.sub.0 (for a typically conventional radome construction). Grating lobes are undesirable lobes, radiation, which occur due to interference when electromagnetic radiation at a suitable frequency meets or transmits through a surface with a periodic and symmetric pattern.
The frequency of the radiation at which the grating lobes occur depends on the packing density of the periodic pattern. If the elements lie more sparsely than .lambda./2, grating lobes occur in the radome's radar signature. That means that the grating lobes will occur for electromagnetic radiation with a wavelength .lambda. which is less than two times the distance between the elements.
In other words, the electromagnetic protection (shielding) which the FSS-radome gives to the sensor is limited. The sensor is electromagnetically unprotected at frequencies which are multiples of the sensor's own frequency and additionally the sensor's radar signature is worsened in that the grating lobes, which were perhaps not present previously, may be introduced at the sensor's own frequency or near to it. The electromagnetic protection which the radome provides is for incident radiation with frequencies lower than the sensor's own frequency, for these frequencies the FSS-surface appears approximately as a purely metallic surface.
Problems with multiple transmission windows present themselves, inter alia, if the FSS-radome is constructed for reasons of radar signature, i.e. if one of the purposes of the radome is to protect sensors which lie behind it from being seen. Another occasion where multiple transmission windows are a problem is if the purpose is to protect the sensor's electronics which lie behind it from both interfering and destructive signals. The problem with grating lobes is only coupled to the desire to reduce/alter the radar signature, on the condition that these do not already occur at an undesired resonant frequency, in which case even the sensor's/antenna's own characteristics are disturbed.
The method of using a periodic aperture pattern in a frequency-selective surface/radome is described in published articles. The method of combining a plurality of layers of similar pattern in order to obtain different characteristics in the FSS-surfaces is also described.
U.S. Pat. No. 5,208,603 discloses a solution, in which an outer layer consisting of periodic patterns of apertures is combined with an inner interposed layer consisting of a periodic pattern of conductive elements. The purpose according to the patent is to obtain a compact radome solution which transmits at two frequencies. The purpose of the middle layer consisting of conductive elements is to achieve a coupling between the aperture layers, which thereby allows transmission at two frequencies. The described solution does not solve the problem of the radome being open to multiple frequencies of the undesired transmission frequencies. The construction allows a denser packing of the elements than with a conventional solution, which means that the grating lobes can be avoided at f.sub.0. But since the layer which is the interface to the surroundings consists of apertures and is resonant for an undesired transmission frequency f.sub.0, the packing density of the apertures will however be such that the grating lobes will occur from about 2f.sub.0.
In British patent GB 2 253 519 a solution is described having an FSS-surface consisting of densely packed layers with periodic patterns of elements which can be apertures and/or conductive elements. The purpose is to obtain a surface, the transmission/reflection characteristics of which can be altered by the layers being displaced relative to one another. The layers are constructed for transmitting or reflecting at a given frequency which can be changed by the layers changing position with respect to each other. There is nothing in the method which prevents transmission at multiple frequencies or the occurrence of grating lobes. The solution which is presented in the British patent is not intended to solve these problems and neither does it do this.